The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to determining prescan phase corrections for a fast spin echo (FSE) acquisition.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Fast spin echo imaging is one of the most widely used MR imaging techniques for clinical applications. Fast spin echo imaging is often preferred because it is tailored to acquire T2, proton density, or T1-weighted images in a relatively short period of time. In this regard, FSE imaging is commonly used for neuro, body, and extremity studies.
Fast spin echo acquisitions are based on the so-called RARE (rapid acquisition with relaxation enhancement) technique. With a FSE pulse sequence, multiple echoes within an echo train are phase encoded. That is, FSE imaging techniques use multiple phase encode steps in conjunction with multiple 180-degree refocusing pulses per repetition time (TR) interval to produce a train of echoes. Typical FSE acquisitions sample an echo train from four to thirty-two echoes and, as such, can achieve a scan reduction factor of four to thirty-two compared to conventional spin echo sequences. With FSE imaging, each echo of the echo train experiences differing amounts of phase encoding that correspond to different lines in k-space. While advantageously reducing scan time and thereby increasing subject throughput, FSE images are prone to excessive ghosting, particularly in shoulder, spine, and head images.
A number of techniques has been developed to reduce ghosting in FSE images. Currently, the primary technique involves the use of a FSE prescan (FSEPS) phase correction. The conventional FSEPS technique acquires non-phase encoded echo data in a prescan of the subject. The phase profiles of the first two echoes in the prescan echo train are then compared to one another to determine a phase correction coefficient or value. This phase correction coefficient is then used to modify the relative phase of the 180 degree refocusing pulses as well as the area of the readout prephaser gradient of the impending FSE scan to improve phase coherence among all the echoes of the echo train of the FSE acquisition. While effective in reducing ghosting in FSE images, this conventional FSEPS phase correction technique is limited in the degree by which ghosting can be reduced. Specifically, since the first two echoes are automatically selected, current FSEPS phase correction fails to account for ghosting levels being highly dependent upon the lope echo of the echo train. The lope echo corresponds to that echo of the echo train that receives zero phase encoding during the FSE acquisition.
It would therefore be desirable to have a system and method capable of FSE phase correction that considers the relationship between the lope echo of an echo train and ghosting levels.